Generally, in receivers which receive broadcast waves such as AM broadcast and FM broadcast, a superheterodyne system is adopted as a receiving system. The superheterodyne system is a receiving system which converts a received broadcast signal into an intermediate frequency signal, which has a fixed frequency independent of an frequency of a received signal (receive frequency), by mixing a predetermined local oscillation signal to the received broadcast signal, and reproduces a sound signal by performing detection processing, amplification, etc. after that, and has a feature that it is superior to other receiving systems in sensitivity, selectivity, etc.
FIG. 8 is a diagram showing the structure of a conventional receiver which adopts the superheterodyne system. The conventional receiver shown in this diagram is constituted by including an antenna 200, a high frequency receiving circuit 202, a local oscillator 204, a mixing circuit 206, an intermediate frequency amplifier circuit 208, an MPU 210, memory 212, a control unit 214, and a digital-to-analog converter (DAC) 216.
In the conventional receiver, data showing the relation between a tuned voltage applied to the high frequency receiving circuit 202 and a received frequency is stored in the memory 212. The MPU 210 calculates data necessary for generating a tuned voltage on the basis of the data stored in the memory 212 to input it into the DAC 216. The tuned voltage which has a desired value is generated by this DAC 216, and is applied to the high-frequency tuning circuit 202.
FIG. 9 is a graph showing the contents of the data stored in the memory 212. As shown in this graph, let a variable range of the received frequency be f0 to f5, and in this variable range, for example, tuned voltages V0, V1, V2, V3, V4, and V5 corresponding to some received frequencies f0, f1, f2, f3, f4, and f5 are measured beforehand, and input data to the DAC 216 necessary for generating these plural tuned voltages is stored in the memory 212. Then, in the case of setting the received frequency of the high frequency receiving circuit 202 as a value other than f0, f1, f2, f3, f4, and f5 which are mentioned above, the MPU 210 obtains input data necessary for generating a desired received frequency by reading the input data of the DAC 216 corresponding to two received frequencies in the vicinity of it from the memory 212 and performing linear interpolating operation to input this into the DAC 216. Thus, a predetermined tuned voltage is applied to the high frequency receiving circuit 202 from the DAC 216, and the desired received frequency is set.
By the way, in the case of setting a tuning frequency of the high frequency receiving circuit 202 with interlocking with an oscillation frequency of the local oscillator 204 by using the conventional system mentioned above, there have been problems: (1) a tracking adjustment takes time, (2) temperature compensation is difficult, and (3) it is weak to the fluctuation of a supply voltage.
As mentioned above, in order to set a suitable tuned voltage by using the DAC 216, it is necessary to perform the tracking adjustment of measuring beforehand a plurality of tuned voltages V0, V1, V2, V3, V4, and V5 as shown in FIG. 9. For example, to measure a tuned voltage V0 is to obtain the tuned voltage V0 at which a tracking error becomes at a minimum, by changing a value of input data of the DAC 216 in the state of outputting a local oscillation signal at a frequency corresponding to the tuning frequency f0 from the local oscillator 204. Usually, it is measured by using a distortion meter and a level meter whether a tracking error is at a minimum, and the distortion rate measurement using the distortion meter takes time of about 10 to 20 seconds for waiting for the stability of an output value. Since such measurement is required every tuned voltage, it takes much time for the tracking adjustment.
In addition, generally in the high frequency receiving circuit 202, since characteristics of devices used change with temperature, a tuning frequency changes with temperature even if a tuned voltage outputted from the DAC 216 is constant. On the other hand, since the local oscillator 204 generally has the phase synchronous loop (PLL) structure of including a voltage controlled oscillator and a variable frequency divider, the frequency of a local oscillation signal determined by a division ratio of the variable frequency divider does not change even if characteristics of devices used change with temperature. Thus, since only the tuning frequency changes with interlocking with a temperature change but the frequency of the local oscillation signal does not change, a tracking error increases in connection with the temperature change. Although it is necessary to equip with a temperature compensation circuit newly in order to avoid such inconvenience, it is not easy to prevent the increase of the tracking error by performing temperature compensation over the entire range of the tuning frequency, and further, there newly arises a problem that a circuit scale become large.
Furthermore, when a supply voltage of the receiver shown in FIG. 8 fluctuates, for example, when a drive voltage drops in a pocket receiver driven by a battery, a car radio driven by an vehicle-mounted battery, or the like, an output voltage of the DAC 216 becomes low with interlocking with the drop of the supply voltage, and hence, a tracking error becomes large since a tuned voltage drops even if the MPU 210 intends to set a desired tuning frequency.